I don't know of any built-in SASS or Compass feature for this, but it's not too hard to add a custom Ruby function to do it. (See the section entitled "Adding Custom Functions" in the SASS reference here.)

Your file with the Ruby function would look like this (let's call it "timestamp.rb"):

And you'd reference the new timestamp function in your SASS file like this:

/*!
* CSS Compiled on: #{timestamp()}
*/

You just need to make sure your "timestamp.rb" file is loaded when you compile the SASS, either by requiring it from a Compass config file, or by using the --require parameter with the SASS command line. When all is said and done, you should get output like the following: